P.M. Kelly, L.R. Francis Rose/ Progress in Materials Science 47(2002)463-557 Although the detailed mechanisms of transformation toughening are more com lex than this simple description and may vary from material to material, the reli- ance on the strains associated with the transformation is universal. without these transformation strains there would be no possibility of a stress-induced transformation-no transformation, no transformation toughening. In addition the transformation strains or more correctly the energy they absorb or the degree of crack-tip shielding they produce-gives rise to the observed toughening. So the whole topic of transformation toughening is dominated by a phase transformation that is associated with a change of shape and /or volume 1. 2. Where do martensitic transformations fit in? A martensitic transformation is a change in crystal structure(a phase change)in the solid state that is athermal. diffusionless and involves the simultaneous. co- operative movement of atoms over distances less than an atomic diameter, so as to result in a macroscopic change of shape of the transformed region [5-11]. The first requirement for a transformation that could lead to transformation toughening is this diffusionless character. If nothing more than small"shuffles"or co-operative atom movements are required, without the need to"reconstruct the crystal struc ture, then the transformation can proceed at a speed approaching that of the velo- city of sound in the crystal [10]. Martensitic transformations satisfy this requirement. However, this alone is not enough. The other requirement is associated with the change of shape -the displacive character of the transformation. It is usually postulated that martensitic transformations are a subset of the overall class of diffusionless, displacive transformations [9, 12, 13]. What is seen as distinguishing a martensitic transformation from other diffusionless, displacive transformations is that the shape change- the displacive component- is relatively large and domi- nated by shear, as opposed to the normally small volume changes. Only in a true martensitic transformation is the resulting shape change sufficiently large that the associated strain energy exerts a dominant influence on the transformation. This is very succinctly expressed in the definition put forward by Cohen et al. [9]: "A mar tensitic transformation is a lattice-distortive, virtually diffusionless structural change having a dominant deviatoric component and associated shape change such that strain energy dominates the kinetics and morphology during the transformation. " In terms of the requirements for transformation toughening outlined above, the martensitic transformation is absolutely ideal. The diffusionless nature ensures a high-speed transformation and the dominant deviatoric strain means that the transformation is readily stress-induced Diffusion-controlled, reconstructive trans- formations, even if they exhibit a shape change, would be far too slow to lead to transformation in time to effect a growing crack. At the same time, rapid diffusion- less transformations that only minor displacive strains are of little use because they will show a ability to be stress-induced. So the two unique features of a martensitic mation- high speed and a change of shape of he transformed volume are both essential if transformation toughening is to*                   ( $+   $   $              (         '          A           '    $       (                    ,                          ( $    $             0  '   $               $         '     $ 9    <-- 9  !    > = *              3 $ 4         O          ( $                          $    $        M76 N
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